1. Field of the Invention
This invention relates to a local area network and more particularly to such a network that can be expanded or contracted.
2. Description of the Prior Art
Prior art networks of terminals or stations usually have been controlled by a master computer or controller which receives transmission requests from the respective stations and grants access to a transmission channel by the individual stations when the channel is available and according to some priority arrangement. Such master controllers add to the cost of the network and are not required where the stations need communicate only between themselves or with a common storage file. Thus, it is desirable to have a network of stations where the transmission control is embedded in or shared by the stations making up that network. Such networks are referred to as "local area" networks.
A particular type of local area network is the so-called "contention" network wherein each station of the network attempts to transmit a message when it is ready to do so and assumes that the message was not received if a corresponding acknowledgment signal is not received after a given period of time. Such a contention network is disclosed in the Malcolm et al U.S. Pat. No. 4,332,027, assigned to the assignee of the present invention.
With such local area networks, it is necessary to provide a communication protocol for the synchronization of message passage between stations of the network which may be adapted to provide different processes. However, no such protocol has yet been devised for the synchronization of the various processes which are distributed to different stations of the network.
Message passage operating systems have been employed for the synchronization of the various processes involved in central processing systems for real-time operation. (See, for example, "Thoth, a Portable Real-Time Operating System", by D. R. Cheriton, M. A. Malcolm, L. S. Melen, and G. R. Sager, CACM, Vol. 22, No. 2, February 1979, pp. 105-115.) However, all of the respective processes are stored in a common memory and the same type of system is not adaptable to a local area network where the processes are distributed across the network. Furthermore, it is desirable to be able to expand the local network or even to connect it to a larger global network and to do so without having to change the addresses of each station in the network.
It is then an object of the present invention to provide an improved communications protocol for local area networks.
It is another object of the invention to provide a protocol for a local area network of stations to allow for the transmission of messages between processes embodied in the different stations.
It is still another object of the present invention to provide for automatic assignment of addresses to various stations in a local area network so that the addition or removal of any station will not affect the operation of the rest of the stations.